Angiosperms flowers: The big picture

  • Sites of sexual reproduction
    • double fertilization


  • Event leading to flowering are complex
    • hormones
    • biological clocks
    • temperature
    • photoperiod


  • Flower production is coordinated with the environment
    • necessary for pollination


  • Dispersal of seeds is the avenue of plant distributions

Angiosperm reproduction: Flowers


  • Flower is a modified stem for reproduction
    • 4 types of modified leaves
    • grouped in whorls (outer → inner)


  • Sepals: leaves at base which enclose flower
    • calyx = all sepals together


  • Petals: colored leaves to attract pollinators
    • corolla = all petals together


  • Two types of fertile organs, stamen and carpel
    • androcieum = male whorl
    • gynociem = female whorl

Flowers - Male structures - androcieum


  • Whorl composed of stamens
    • anther + stalk


  • Anther: where pollen produced
    • each anther has 4 chambers


  • Filament: stalk that holds the anther


  • Pollen: make gametophyte
    • 15-200um
    • generally oval or spherical

Flowers - Female structures - gynociem

  • Whorl composed of carpels
    • middle of flower
    • 1 → many carpels


  • Stigma: receives pollen
    • sticky


  • Style: leads from stigma to ovary
    • where pollen tube forms


  • Ovary: contains ovules
    • ovules becomes seeds
    • 1 → many

Flowers - Female structures - gynociem


  • Pistil also refers to 1 unit
    • carpels can be separate or fused


  • Complexity leads to diverse flowers
    • also leads to diverse fruits

Variation in flower strucutre




  • Complete flower: all four whorls
  • Incomplete flower: missing at least one whorl


  • Perfect flower: both stamens and carpels
  • Imperfect flower: missing one or the other


  • Regular flower = radial symmetry (rose)
  • Irregular flower has bilateral symmetry (iris)

When to flower? Arabidpopsis has many known pathways




  • Photoperiod: long days trigger flowering genes
    • photoreceptor pigments


  • Vernalization: prolonged cold induces flowering


  • Internal hormonal signals
    • inhibition or induction of flowering
    • independent of environment


  • Why so many?

Flowering in marijuana




  • Flowering periods triggered by photoperiodism
    • 6hrs or more of darkness
    • vegetative growth synced w/ summer daylight


  • Longer darkness cause hormonal change
    • triggered by 2 photocrome pigments
    • PR (660-700nm) & PFR (760-800nm)
    • darkness switches PFR to PR
    • tipping point of PFR:PR

Pollination

  • Sex cells must travel from one plant to another
    • sexual partners may be widely scattered


  • Seed plants: pollen carried by wind, insects & birds
    • inevitable pollen loss
    • may also require nectar production


  • Pollination = transfer of pollen from anther to stigma
    • not fertilization!


  • Some flowering plants can self-pollinate
    • stamen & carpel mature @ same time
    • large genetic consequences
    • no nectar necessary

Mechanisnms against self-pollination


  • Many flowers have both male & female structures
    • diecious plant


  • Pollen and the ovary mature at different times


  • Physical flower features that prevent self-pollination
    • styles length relative to the stamens = heterostyly


  • Biochemical blockers to reject closely related pollen
    • e.g. enzymatic destruction of RNA
    • chemical tests (proteins) on pollen tube

Evolution of flower color and smell

Alternation of generations: Where is the gametophyte?



  • Male: microspore inside pollen grain
    • microgametophyte = max 3 cells
    • generative cell → mitosis → 2 sperm
    • vegetative cell makes pollen tube


  • Female: 7 cell embryo sac, inside ovule
    • megaspore develops into megagametophyte
    • mitosis 3x
    • one cell has 2 nuclei
    • one cell = egg


  • Internal fertilization!!!

Angiosperm reproduction: double fertilization

Comparing seed plant reproduction

Fruits develop from ovaries




  • Ovule develop into seeds


  • Stigma, style, sepals & petals wither away


  • 3 layers develop during growth
    • exocarp = outer skin
    • mesocarp = flesh
    • endocarp = stone, pit or thin inner layer

Type of gynociem determines fruit type

Fruits/Seeds as agents of dispersal


  • Seeds capable of long-distance dispersal
    • pros: promotes sexual reproduction
    • cons: diverse landing sites


  • 1 plant = 1,000s of seeds (possible)
    • each a genetic experiment
    • significant cost to plant


  • Fruits/seeds by wind = light, wings or parachutes
  • Fruits/seeds by water = buoyant & anti-rot
  • Mechanisms to stick to animals
  • Edible but not digested